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When dinosaurs ruled on land, these reptiles prowled the seas

For millions of years, reptiles dominated the Earth. Many that dwelled on land were dinosaurs. But no dinos swam in the seas. The oceans had their own cadre of reptiles. Many were top predators, the sharks and killer whales of their time. And they would have made the oceans very dangerous.

Some of these marine reptiles were shaped like dolphins and probably could swim fast. Some were as large and as long as a school bus. But they lacked the distinctive hip structure that only dinos had.

A dinosaur had distinctive holes in its pelvis where its thighbones were attached, notes Sterling Nesbitt. He’s a vertebrate paleontologist at Virginia Tech in Blacksburg. Marine reptiles of the same time period lacked such holes.

About 252 million years ago, there was a mass extinction. At that time, huge volcanoes erupted in what is now Siberia. The ocean’s chemistry changed as well. As a result, large numbers of animals, plants and other species died out. Overall, about 90 percent of ocean species and 70 percent species on land disappeared. After the devastated ecosystems recovered, the few species that survived evolved to better fit in with the new environmental conditions.

With so many ocean species gone, some land creatures tried an aquatic lifestyle — and succeeded. These animals evolved to become ichthyosaurs (IK-thee-oh-saurs). Much later, after additional mass extinctions, other land-dwelling reptiles took to the seas. Their descendants evolved to become plesiosaurs, pliosaurs and mosasaurs.

People have been unearthing fossils of such sea creatures for hundreds of years. But scientists are still finding new species and discovering new information on what these animals looked like and how they lived.

Fish-lizards of the sea

Ichthyosaurs were among the earliest lizards to take to the seas. Their name even means “fish-lizard” in Greek. On the whole, ichthyosaurs were very successful. So far, paleontologists have discovered and named more than 100 different species of them, notes Benjamin Moon. He’s a vertebrate paleontologist at the University of Bristol in England.

Species from this group lived from about 248 million years ago to about 95 million years ago. Their fossils have been found worldwide. None of these came from rocks that started out as sediments from lakes or rivers, he notes. So ichthyosaurs all must have been ocean dwellers. Some of these aquatic reptiles were no more than 80 centimeters (about 31 inches) long. Others spanned a whopping 22 meters (72 feet). Some were very streamlined, like today’s dolphins. Others had more lizard-like proportions.

Some ichthyosaurs lived and foraged in coastal waters at the edge of continents. But others apparently swam in the open ocean, far from land. They even gave birth to live young at sea, like today’s whales and porpoises do. This is an example ofconvergent evolution, or the development of similar features in totally unrelated lineages. These similarities likely evolved from having to adapt to similar environments or places within an ecosystem.

Paleontologists had long suspected that some ichthyosaurs dove deep to find prey, like modern-day sperm whales. One of these animals was Ophthalmosaurus (Op-THAHL-moe-saur-us). With eyes up to 10 centimeters (4 inches) across, it takes its name — “eye lizard” — from the Greek. These 6-meter (nearly 20-foot) long creatures must have been chasing prey into very deep, dark waters, some scientists believe. Others have suggested those big eyes would have let the lizards hunt at night.

A recent study of some amazingly preserved fossils may help put an end to the debate. Scientists unearthed the fossils from rocks that are between 190 million and 196 million years old. Most fossils preserve just bone and other hard tissue. But these fossils included soft tissues that are probably skin.

Peppering the inside of that apparent skin were tiny blob-like structures. These measured between 500 and 800 nanometers long. That’s the same size as the pigment-carrying structures in skin cells and feathers of today’s mammals and birds, notes Johan Lindgren. He’s a vertebrate paleontologist at Lund University in Sweden. He and his colleagues now propose that the tiny blobs in this reptile are the remains of its pigment-carrying structures. Lindgren’s team described the findings in the February 27, 2014 issue of Nature.